Mobility Equation¶
The mobility equation describes the drift (terminal) speed (vt) of a particle throuh some medium (e.g. high pressure gas) in the presence of an electric field (E) with magnitude E:
The constant of proportionality k is called the mobility constant and is specific to the type of the particle, such as its collision cross section, and the type of the medium (such as gas chemical composition, pressure and temperature).
The fact that different particles have different mobility constants is used to good effect in ion mobility spectrometry (IMS) to separate ions.
Although k depends on gas pressure P and temperature T, these effects can be removed by re-expressing k as a reduced mobility, which is the mobility at standard temperature and pressure (STP) conditions:
Here, n is the number density of background gas particles under current conditions (n) and standard conditions (n 0). n can be related to P and T via the ideal gas law: PV = nRT. Note, however, that the linearity between k and k0 may only be approximate within certain limits, and the ideal gas law might not hold exactly.
k is not always a constant but could depend on E. This is particularly an issue at high fields, which is used to good effect in high-field asymmetric-waveform ion mobility spectrometry (FAIMS). See the SIMION Example: faims documentation for details.
SIMION Specific¶
These SIMION examples relate to mobility:
- SIMION Example: drag - Stokes’ law drag force calibrated to match mobility
- SIMION Example: collision_sds - combined mobility and diffusion collision model, for high pressures
- SIMION Example: collision_hs1 - hard-sphere collision model, for low pressures (free molecular), and is not a mobility model.
- SIMION Example: ims - simple ion mobility example
- SIMION Example: faims - FAIMS example, based on extension to the SDS model, with detailed FAIMS documentation
